In the conventional art, a fuel supply to an engine is interrupted and the engine is disconnected from a powertrain to improve fuel economy, under a driving condition where neither a driving force nor a braking force is required. For example, Japanese Patent Publication No. 5141802 describes a hybrid drive system configured to stop the engine while disconnecting from a powertrain while running. According to the teachings of Japanese Patent Publication No. 5141802, a rotational speed of the engine is controlled by a first electric motor, and a second electric motor is operated by an electric power generated by the first electric motor. A power generated by the second electric motor is transmitted to an output shaft in addition to a power of the engine. The engine is connected with an overdrive mechanism through a clutch. The overdrive mechanism is a double pinion planetary gear mechanism in which a sun gear is halted, a carrier is connected with the output shaft of the engine through the low clutch, and a ring gear is connected with the output shaft through the high clutch.
In the hybrid drive system taught by Japanese Patent Publication No. 5141802, the carrier of the overdrive mechanism is connected with a carrier of a single-pinion planetary gear mechanism serving as a power distribution mechanism, and the first electric motor is connected with the sun gear of the power distribution mechanism. A ring gear of the power distribution mechanism serves as an output element, and the second electric motor is connected with the ring gear through an output side transmission.
Accordingly, in the hybrid drive system taught by Japanese Patent Publication No. 5141802, the engine is connected selectively with the carrier of the power distribution mechanism by engaging the low clutch or the high clutch. In this situation, given that a speed of the sun gear is varied by the first electric motor, the engine speed is changed in accordance with a speed of the ring gear governed by the vehicle speed and the speed of the sun gear. That is, the engine speed is controlled by the first electric motor. In this situation, the first electric motor may serve as a generator, and a generated electric power is supplied to the second electric motor to generate the driving force.
Given that the low clutch is engaged, the above-mentioned overdrive mechanism outputs an engine torque transmitted to the carrier thereof to the power distribution mechanism from the carrier. That is, a direct stage or a low stage is established in this situation. By contrast, given that the high clutch is engaged, the ring gear serves as an input element, the sun gear serves as a fixing element and the carrier serves as an output element. That is, an overdrive stage or a high stage is established. This means that the overdrive mechanism serves as a speed increasing mechanism. Provided that both of the low clutch and the high clutch are disengaged, the engine is disconnected from a downstream side of the overdrive mechanism. As described, the second electric motor is connected with the ring gear of the power distribution mechanism for transmitting the power to driving wheels. Therefore, the hybrid vehicle can be driven by supplying the electric power to the second electric motor even if the engine is disconnected from the powertrain. That is, the hybrid vehicle is driven under an EV mode.
As described, a power loss in the hybrid vehicle resulting from rotating the engine concurrently can be reduced by disengaging the clutches under the EV mode, that is, under the situation that neither the driving force nor the braking force of the engine is not especially required. Consequently, fuel efficiency (i.e., fuel consumption rate) and electric efficiency (i.e., electric consumption rate) are improved. Such control for disconnecting the engine from the powertrain may also be carried out in a vehicle other than the hybrid vehicle to improve the fuel efficiency if the engine can be disconnected from the powertrain in the vehicle.
The engine being stopped and disconnected from the powertrain is expected to be restarted while running the vehicle. However, there may be a situation in which an engagement device and a motor-generator or a motor may not be controlled as desired when restarting the engine. In this case, a shock may be caused if the engaging device is engaged completely after raising the speed of the engine. Otherwise, the shock may also be caused if the engaging device is engaged out of synchronization.
The present invention has been conceived nothing the foregoing technical problems, and it is an object to provide a vehicle control system for reducing shocks resulting from restarting an engine disconnected from a powertrain while the vehicle is running.